Wash sweetbreads well in cool water.
Cover with water, adding 1 teaspoon salt and 1 tablespoon lemon juice per quart of water.
Bring to a boil and simmer about 15 minutes.
Drain and plunge sweetbreads immediately into cold water to stop cooking; drain again.
With your fingers and the point of a sharp knife, remove as much of the white connecting membrane as possible and break sweetbreads into bite-sized pieces. (If you are not going to finish cooking them immediately, place them in a bowl of cold water and refrigerate; use within 36 hours.) Fry bacon until crisp; drain, crumble, and set aside for garnish.
Melt butter (or use drippings) in a frying pan and saute onion and mushrooms just until tender.
Add sweetbreads, salt, tarragon, wine, and cream.
Boil rapidly, stirring occasionally, over high heat until sauce is reduced by half and slightly thickened.